1. Field of Invention
This invention relates to a dual fuel engine, and more specifically to a method for converting a conventional diesel engine to one which is capable of receiving both diesel and at least one second fuel, which is most commonly liquid petroleum gas (LPG) or less commonly compressed natural gas (CNG). The invention also relates to and is to be considered as extending to a dual fuel engine or multi-fuel engine which operates in a particular manner as hereinafter described.
2. Background of Invention
Although the following description is provided with exclusive reference to the conversion of conventional diesel engines to engines which are capable of receiving both diesel and LPG as the main combustive fuel components, the skilled reader will appreciate that the following invention can easily be adapted to allow for conversion of conventional diesel engines to permit them to utilize a wide variety of different gases, and therefore the invention should not be considered as solely limited to dual fuel or diesel/LPG engines.
Dual fuel engines have been produced in the past, and indeed it has for a long time period been known that the combination of diesel fuel with LPG inside the cylinders of an engine not only can improve efficiency of the engine, but automatically reduces the overall fuel costs of operating the engine. For example, in the UK, diesel fuel typically currently retails for around £0.76 per litre, whereas LPG can readily be obtained for around £0.23 per litre. However such cost savings must be offset against the cost of converting the diesel engine.
Currently, it is generally the case that most conventional diesel engine conversions are time consuming, elaborate and expensive. As the reader will be aware, diesel engines are not provided with spark plugs as the ignition of the diesel fuel is achieved solely by pressure developed in the cylinders by virtue of the reciprocating motion of the pistons and additionally by virtue of the inherent heat of the engine during operation. It is this latter requirement for heat that necessitates a delay between initially starting the engine (which causes a heater to operate) and firing the engine, i.e. causing the driveshaft on which the pistons are mounted to rotate.
In replacing some of the diesel which is injected into the cylinders with an amount of LPG, it is necessary to remove the diesel injectors and the diesel injection pump and fit reduced capacity components in their place, change the head gasket of the engine to reduce the compression ratio achieved in the cylinders, and fit a distributor to adjust the timing of the engines. A conventional conversion, particularly for a large diesel engine such as might be provided in a locomotive or articulated lorry may cost in the region of £28,000. Of course this figure depends on the size of the engine, and in particular the number of cylinders within the engine.
Notwithstanding the expense of diesel engine conversion, the advantages in terms of fuel cost savings and efficiency characteristics achieved by conversion often outweigh the cost, and many conversions have at the present time been conducted. Indeed relevant prior art reflects this fact.
For example, U.S. Pat. No. 4,463,734 discloses a diesel-based engine which is designed to burn a mixture of LPG and diesel. The diesel is injected into the cylinders in the conventional manner, but in addition to this, LPG in its gaseous phase is mixed with the airflow which is essential to the proper functioning of the engine. In general, engines may be normally aspirated, in which case the air is forced into the engine simply by virtue of the forward motion of the vehicle in which the engine is mounted, or alternatively the engine may be turbocharged in which case a small turbine driven by the exhaust gases from the cylinder is linked to a compressor to compress the air within the inlet manifold before it is delivered into the engine cylinders. This results in improved performance and additionally enhances the efficiency of the engine. Regardless of the condition of the air when it is introduced into the engine, the air is essential as it provides a supply of oxygen to fuel the combustion. In the abovementioned U.S. patent, the LPG is mixed with the incoming air so that the inlet manifold to the engine contains both a supply of oxygen and gaseous LPG. This mixture is then introduced into the cylinders of the engine in the same way that a conventional air stream would be introduced, namely through the air inlet valves of each cylinder.
As mentioned above the diesel continues to be injected into the cylinders through specifically adapted injectors provided on each cylinder.
The U.S. patent goes on to describe how the governor of the diesel engine is coupled to an LPG regulator so that as the load on the engine increases, the percentage of total fuel delivered to each cylinder of the engine which is LPG increases significantly quicker than does that percentage of total fuel which is diesel, this latter increase being achieved under the conventional operation of the engine and not being regulated in any way by the governor. Specifically, the inventor mentioned in connection with said U.S. Patent the realisation that when the engine was idling or operating under very light loads the amount of diesel as expressed as a percentage of the total fuel weight (i.e. diesel and LPG) introduced into the cylinder should be relatively high, typically in the region of 75%, whereas when the engine was operating under heavy or full load conditions, the amount of diesel expressed a percentage of the total fuel delivered should be quite low, typically of the order of 20%. Under these parameters, the engine could operate significantly more powerfully, more efficiently, and at lower cost.
However, a fundamental disadvantage of the invention described in this patent is the predetermined nature of the operating characteristics necessitated by the direct mechanical link between the LPG regulator (which is a valve in the main embodiment) and the governor of the diesel engine. For instance, under a particular load, the governor of the engine will necessarily operate in a particular manner causing the regulator to open or close to a predetermined extent. Accordingly, the system proposed in the U.S. Patent is inflexible.
A further disadvantage of this invention is the fact that the LPG is mixed with the incoming air flow in a plenum ring prior to entry of the air into the cylinders of the engine. This further precludes versatile operation of the engine.